1. Field of the Invention
The present invention relates to a method for measuring Brillouin backscattered light from an optical fibre used for optical time domain reflectometry, the method utilising digitisation. The invention further relates to apparatus for performing the method.
2. Description of Related Art
Optical time domain reflectometry (OTDR) is a technique that uses optical fibre to make remote measurements of various parameters. A probe pulse of light is launched into an end of a fibre that is deployed through a region of interest, for example down an oil well. The pulse propagates along the fibre, and part of the light is backscattered from points along the length of the fibre and returns to the launch end, where it is detected. The propagation time to the scattering point and back is recorded as the light returns, so the location of the scattering point can be calculated using the speed of propagation in the fibre. Also, various physical parameters such as temperature, strain, and pressure have an effect on how the light is scattered, including producing Raman and Brillouin frequency shifts. The value of the parameters can be calculated from the size, width, and intensity of these frequency shifts. Thus, by making the appropriate conversion from time to distance, a map of the distribution of a physical parameter along the fibre length can be obtained.
In Brillouin-based OTDR, one or more Brillouin lines are measured in the scattered light spectrum. These lines are shifted in frequency from the frequency of the probe pulse. From a measured Brillouin spectrum, one can extract at least the intensity of the line or lines and the size of the frequency shift, and use this information to determine physical parameters along the fibre.
Conventionally, Brillouin signals have been measured by direct detection, where the Brillouin light is incident directly on a photodetector, or by heterodyne detection, in which the Brillouin signal is mixed with a signal from a local oscillator and the resulting difference frequency signal is passed for detection.
One measurement technique uses optical discrimination, in which an optical filter switches light between the two arms of a Mach-Zehnder interferometer, and an estimate of the central frequency of the Brillouin line is obtained from the relative intensities of the optical signals emerging from each arm [1]. A similar procedure relies instead on electrical discrimination [2].
A problem with discriminator-based techniques is the need to employ a wide input frequency spectrum to capture the full range of potential output signal frequencies. The necessary broad bandwidth tends to degrade performance.
Other techniques are based on frequency scanning and recording an intensity/time signal for each scan position. For example, one may scan an optical filter across the expected frequency spectrum before passing the filtered light to a detector. The optical filter may be a Fabry-Perot interferometer that is scanned slowly compared with the pulse repetition frequency of the probe pulses. For each pulse a series of intensity measurements is made as a function of time/distance along the fibre, and may be further averaged over several pulses. A series is recorded for every position of the filter, from which a Brillouin spectrum for each location along the fibre can be constructed [3].
An alternative approach [4, 5] uses a microwave heterodyne method, in which the backscattered light is mixed on a photodiode, thus creating a beat frequency spectrum that shifts the information from the optical domain to the microwave domain. An electrical local oscillator is scanned in frequency and a microwave receiver section passes a fixed intermediate frequency that is further amplified, filtered and detected, thus creating a quasi-DC signal. The latter provides an indication of the power within the bandwidth of the system as a function of position along the fibre.
For these various scanning methods, the data acquisition time is typically slow, since the signals must be averaged in two dimensions. Depending on the sampling interval in the frequency offset domain and the span of frequencies to be covered, this can be a lengthy process during which essential but sparsely used information is acquired. A large frequency range must be looked at for each position along the fibre to ensure that the shifted frequency is found, but the frequency line itself occupies only a small part of that range. Measurements outside the line must be made to locate the line, but contain no information regarding the parameter being measured. In contrast, the discrimination methods, at least in the electrical domain, require an acceptance bandwidth that is much wider than the Brillouin signal to allow for the possible range of the frequency shift and thus suffer from increased noise.